Miniaturization of electronic components has led to various improvements in semiconductor technology to make electronic components ever-smaller. Such components may include simple components such as diodes, up to complex components such as integrated circuits. Apart from electronic components, mechanical components can also be manufactured using the same technology.
In the art of semiconductor technology, it is commonly known that a wafer of semiconductor material, typically silicon, is processed to form the components in a surface area of the wafer. The wafer is macroscopic, having a diameter ranging in the order of 20-300 mm, while the components are microscopic, typically having a size in the micrometer range. Each component, or complex of components, is made in a small wafer portion, with the various wafer portions being located at a small distance from each other. After the processing steps, the wafer is cut to separate the various wafer portions from each other, so that the components become available independent from each other. After separation, each separated wafer portion is referred to as a die, and the separation process is known as dicing. The present invention relates particularly to the field of wafer dicing.
The various wafer portions are typically arranged in a matrix pattern, separated by mutually orthogonal lanes, also indicated as “dicing streets”. The separation process involves applying a cut in each dicing street. Evidently, it is desirable that the surface area of the wafer is used as efficiently as possible, therefore said dicing streets are very narrow, which makes the precision requirements for the dicing processing very demanding.
The cutting is executed in a cutting apparatus, that comprises a holder for holding the wafer and a cutting blade for cutting the wafer. In operation, the cutting blade makes the cut along a straight line in a cutting direction. For assuring that the cut is made in the correct position, the wafer must be accurately aligned with the cutting blade and the cutting direction. This involves the step of aligning the dicing streets of the wafer with reference directions of the cutting apparatus. These reference directions will typically be mutually orthogonal directions indicated as X and Y directions, but the scope of the present invention is not restricted to these details, and in principle it is possible that the wafers are cut along three or more different directions.
For aligning the dicing streets of the wafer with reference directions of the cutting apparatus, it is common practice to use a visual imaging system. A camera is arranged to view the wafer on the holder, and an image of the wafer (or a wafer portion) is displayed on a display screen. Reference lines that represent the said reference directions are also displayed on the display screen. In an initiation mode, a human operator manually manipulates the wafer such that the dicing streets displayed on the screen are accurately aligned with the reference lines on the screen. When the human operator is satisfied with the alignment, an image is taken of the wafer, and this image is stored in a data memory of the cutting apparatus.
In an operation mode, the cutting apparatus receives a new wafer to be cut. The cutting process essentially comprises three steps.
A first step is an alignment step, in which a control device fully automatically controls the holder to move the wafer (linear displacement and/or rotation) to such position that the instantaneous wafer image accurately corresponds to the stored image.
A second step is a determination step in which the exact positions of the dicing streets are determined by the control device. The dicing streets which extend in the X-direction (indicated as X-streets) are mutually parallel and thus have a mutual distance in the Y-direction (indicated as Y-pitch), which is equal for all pairs of X-streets. Likewise, the dicing streets which extend in the Y-direction (indicated as Y-streets) are mutually parallel and thus have a mutual distance in the X-direction (indicated as X-pitch), which is equal for all pairs of Y-streets. The X-pitch and the Y-pitch are known to the control device, or in any case the control device has information that defines approximation values for the X-pitch and the Y-pitch, but in practical conditions there may be slight variations. Thus, the control device controls the holder to displace the wafer in steps corresponding to the X-pitch and the Y-pitch, then, with the camera viewing the wafer, the control device performs an accurate alignment of the wafer so that the instantaneous wafer image again accurately corresponds to the stored image, and the thus obtained precise coordinates of the X-streets and Y-streets are stored in memory.
A third step is the actual cutting step. The control device controls the holder to displace the wafer to the positions determined in the second step and retrieved from memory, and performs the cutting operation.
It is to be noted that in a high-quality wafer, immediately after manufacture, the dicing streets are mutually parallel and have very accurate pitch. Thus, when the wafer has been correctly aligned in the cutting apparatus, the position of an X-street can be characterized as an Y-coordinate and the position of an Y-street can be characterized as an X-coordinate. Further, with a constant pitch over the wafer surface, it is not necessary to know all individual Y-coordinates of the X-streets and to know all individual X-coordinates of the Y-streets, since stepping from one X or Y street to its neighbour only requires to take a step equal to the pitch in the Y or X direction, respectively.
The known process as described above works well in practice. However, an essential feature of the known process is that in the determination step, the positions of dicing streets are determined somewhere in the central area of the wafer, and this requires that the dicing streets of the wafer are exposed to visual observation by the camera of the visual imaging system.
A special type of semiconductor dice is the so-called “molded dice”. Their application is typically in the area of semiconductor packages, where two or more dice are stacked upon each other. These dice are provided with a non-conductive coating, containing for instance a plastic or an epoxy, and contact terminals that extend through the coating to contact the die. Manufacturing first involves separating the individual dice, such as by the prior art process as described above. Then a molding compound application step is performed, in which the molding compound is applied to the dice, typically in liquid form. The molding compound covers the top surface of the dice, and fills the cut open dicing streets between the dice. Then the molding compound solidifies or hardens.
In this stage, the molded dice together with the molding compound have the shape of a disc, which is termed a “molded wafer”. Now the individual dice must be separated again, this time by cutting the molding compound that has filled the dicing streets. The cut to be made must follow the dicing streets; if it is desired that this cut be substantially narrower than the dicing streets, the cut is made by a laser.
The task to be performed, cutting the wafer, seems comparable to the task described above. It is however not possible to use the technique described above, because the upper side of the wafer is now covered with non-transparent molding compound and hence the dicing streets of the wafer are not available for visual observation by the camera of the visual imaging system.
Some prior art methods rely on visual indications visible to above the molding compound, and on an assumption that the dicing streets are arranged according to a perfect grid. However, on the one hand, alignment on the basis of such visual indications has already a limited accuracy, and on the other hand, while the dicing streets were indeed arranged according to a perfect grid on manufacture, this assumption does not hold true any more after the steps of cutting and applying the covering molding compound. Thus, these prior art methods lack accuracy.
As a partial solution to the above problem, it has already been proposed to remove a circumferential edge area of the molding compound, such as to expose the underlying end portions of the dicing streets. While this involves damaging and hence sacrificing the corresponding circumferential edge area of the wafer, this is in practice acceptable.
This prior art method allows for examining the wafer for finding the positions of the dicing streets. Better said, this prior art method allows for finding the end portions of the dicing streets. The dicing streets themselves may be considered to extend as respective straight lines between these end portions. However, this requires an accurate pairing of opposite end portions; even a slight mismatch will cause cuts to be made in an oblique manner and may ruin the wafer. An accurate pairing requires an accurate determination of position and direction of each dicing street; on the other hand, this determination should be done sufficiently fast, otherwise the throughput would become too low.